Cellular mobile telephone systems rely on the reuse of radio frequencies in different cells or radio coverage areas. When a mobile terminal moves from communication with a base station in one cell (original base station) to another it is necessary to “handover” the mobile terminal to the base station (target base station) of the new cell. Handovers may be passive or active, i.e. a handover may be necessary when there is no active communication and the mobile terminal is “camped” on the current cell and must be transferred to the new cell (passive handover) or when there is an active communication which must be transferred from the current to the target base station (active handover). In a “soft handover” the mobile terminal communicates with both the old base station and the target base station at the same time and the network may decide, based on certain communication qualities or other criteria, when the signals arriving via the target base station are acceptable and the link to the original base station may be broken.
A method and a system for providing a communication with the mobile terminal through more than one base station during the handover process are disclosed in U.S. Pat. No. 5,625,876. Using this system, a communication between the mobile terminal and the end user is not interrupted by a handoff from the original base station to a target base station. The communication with the target base station is established before communication with the original base station is terminated. When the mobile terminal is in communication with two base stations, a single signal for the end user may be created from the signals from each base station by a cellular or personal communication system controller.
In systems in which a mobile terminal may communicate with several base stations at the same time, e.g. CDMA systems, mobile terminal assisted handoff may operate based on the signal strength of beacon or pilot signals from several sets of base stations as measured by the mobile terminal. An Active Set is the set of base stations through which active communication is established. The Neighbour Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot or beacon signal strength of sufficient level to support communication of adequate quality. The Candidate Set is a set of base stations having a pilot or beacon signal strength of sufficient level to establish communication.
When communications are initially established, a mobile terminal communicates through a first base station and the Active Set contains only the first base station. The mobile terminal monitors the pilot or beacon signal strength of the base stations surrounding at and each of these is allocated to the Active Set, the Candidate Set, or the Neighbour Set. When a pilot or beacon signal of a base station in the Neighbour Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbour Set of the mobile terminal. The mobile terminal communicates a message to the original base station identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the mobile terminal. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the mobile terminal and a command to establish communications therewith. A message is also transmitted to the mobile terminal through the original base station. The message identifies a new Active Set that includes the original and the new base stations. The mobile terminal searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the original base station. This process can continue with additional base stations.
When the mobile terminal is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbour Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined period of time, the mobile terminal generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the mobile terminal is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot or beacon signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The base station through which communication is established sends a message to the mobile terminal. The cellular or personal communication system controller also communicates information to the relevant base station to terminate communications with the mobile terminal. The mobile terminal communications are thus routed only through base stations identified in the new Active Set. In a cellular or personal communication telephone system, maximising the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is also extremely important.
All messaging across the air interface between a mobile terminal and a base station involves complex digital signal processing and certain parameters of the messaging, e.g. types of spreading codes or forward error coding, data rate, bandwidth, frequency, must be known in advance by the mobile terminals and the base stations before successful communication can take place. Hence, the digital transmissions across the radio link have a certain configuration. Any change of the parameters of this radio link configuration must be communicated to the respective transmitters and receivers before the change may be implemented. When a mobile terminal is in soft handover, the radio links to the plurality of base stations have a common configuration. It is necessary to communicate a change of radio link configuration to the mobile terminal and all the base stations involved in the current communication with the mobile terminal. In particular, when the configuration is changed then there is the danger that any network element which does not recognise the change will no longer be able to decode signals after the change. If this network element is the mobile terminal then connection to the mobile is lost and cannot be recovered other than by rapidly reverting to the old configuration. To maintain synchronisation a radio link configuration change message may be sent from the network to each of the current base stations and the mobile terminal giving details of the changes. The new radio link configuration is not applied immediately but after a certain predetermined time. The delay is chosen so that the on average the mobile terminal 7 and the base stations will have sufficient time to reconfigure. Using a fixed delay time has the disadvantage that this time will normally be set conservatively so that reconfiguration will take longer than necessary on average.
It is an object of the present invention is to provide a method and a telecommunications system implementing the method which reduces the time required to reconfigure transmissions across the air interface.